Pesticide Contamination Prevention Act Review Process Triggered by Detections of Diuron in Groundwater

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Formal Review Process and Regulatory Changes

The formal review process began in July 1986, following the determination that detections of diuron in groundwater were the result of its legal agricultural use. Following a public hearing in April 1987, the PREC subcommittee found that use modifications, including banning applications in recharge areas, reducing rates for non-crop uses, and requiring the use of alternative pesticides, would create a high probability that diuron would not continue to pollute groundwater. In October 1987, the Director concurred with the subcommittee's findings and, among other actions, banned the use of diuron in artificial recharge areas and in Pesticide Management Zones (PMZ) where it had been detected.

  • Notice to registrants, PDF (353 kb)
  • PREC subcommittee "Findings and Recommendations," PDF (5.4 mb)
  • Director's response regarding the continued use of diuron, PDF (174 kb)

Current Regulatory Status

In 2004, DPR expanded the areas where certain pesticides, including diuron, were regulated and adopted mandatory mitigation measures to protect groundwater. Called "groundwater protection areas" (GWPAs), these new areas included all former PMZs as well as sections of land with no reported detections but with soil types and depths-to-ground water that are characteristic of contaminated areas. Based on the pathway of pesticide movement to groundwater, GWPAs are designated as either leaching or runoff. In leaching GWPAs, the mandatory mitigation measures are designed to prevent over-irrigation while in runoff GWPAs, they are designed to either prevent offsite movement of contaminated runoff or manage contaminated runoff so that it does not move to groundwater. In addition, pesticides regulated as groundwater contaminants may not be applied in artificial recharge basins, inside canal or ditch banks, or on engineered rights-of-way throughout California unless applicators observe mandatory mitigation measures.

For more information, please see: Identifying and Protecting Groundwater Protection Areas